RE: [PATCH] serial: xilinx_uartps: Fix race condition causing stuck TX

[Shubhrajyoti Datta <shubhraj@xilinx.com>]

> -----Original Message-----
> From: Michal Simek <michal.simek@xilinx.com>
> Sent: Monday, November 1, 2021 5:58 PM
> To: Anssi Hannula <anssi.hannula@bitwise.fi>; Michal Simek
> <michals@xilinx.com>; Shubhrajyoti Datta <shubhraj@xilinx.com>
> Cc: linux-serial@vger.kernel.org; Greg Kroah-Hartman
> <gregkh@linuxfoundation.org>; linux-arm-kernel@lists.infradead.org; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH] serial: xilinx_uartps: Fix race condition causing stuck TX
> 
> 
> 
> On 10/26/21 12:27, Anssi Hannula wrote:
> > xilinx_uartps .start_tx() clears TXEMPTY when enabling TXEMPTY to
> > avoid any previous TXEVENT event asserting the UART interrupt. This
> > clear operation is done immediately after filling the TX FIFO.
> >
> > However, if the bytes inserted by cdns_uart_handle_tx() are consumed
> > by the UART before the TXEMPTY is cleared, the clear operation eats
> > the new TXEMPTY event as well, causing cdns_uart_isr() to never
> > receive the TXEMPTY event. If there are bytes still queued in circbuf,
> > TX will get stuck as they will never get transferred to FIFO (unless
> > new bytes are queued to circbuf in which case .start_tx() is called again).
> >
> > While the racy missed TXEMPTY occurs fairly often with short data
> > sequences (e.g. write 1 byte), in those cases circbuf is usually empty
> > so no action on TXEMPTY would have been needed anyway. On the other
> > hand, longer data sequences make the race much more unlikely as UART
> > takes longer to consume the TX FIFO. Therefore it is rare for this
> > race to cause visible issues in general.
> >
> > Fix the race by clearing the TXEMPTY bit in ISR *before* filling the
> > FIFO.
> >
> > The TXEMPTY bit in ISR will only get asserted at the exact moment the
> > TX FIFO *becomes* empty, so clearing the bit before filling FIFO does
> > not cause an extra immediate assertion even if the FIFO is initially
> > empty.
> >
> > This is hard to reproduce directly on a normal system, but inserting
> > e.g. udelay(200) after cdns_uart_handle_tx(port), setting 4000000
> > baud, and then running "dd if=/dev/zero bs=128 of=/dev/ttyPS0 count=50"
> > reliably reproduces the issue on my ZynqMP test system unless this fix
> > is applied.
> >
> > Fixes: 85baf542d54e ("tty: xuartps: support 64 byte FIFO size")
> > Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
> > ---
> >   drivers/tty/serial/xilinx_uartps.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/xilinx_uartps.c
> > b/drivers/tty/serial/xilinx_uartps.c
> > index 962e522ccc45..d5e243908d9f 100644
> > --- a/drivers/tty/serial/xilinx_uartps.c
> > +++ b/drivers/tty/serial/xilinx_uartps.c
> > @@ -601,9 +601,10 @@ static void cdns_uart_start_tx(struct uart_port
> *port)
> >   	if (uart_circ_empty(&port->state->xmit))
> >   		return;
> >
> > +	writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_ISR);
> > +
> >   	cdns_uart_handle_tx(port);
> >
> > -	writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_ISR);
> >   	/* Enable the TX Empty interrupt */
> >   	writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_IER);
> >   }
> >
> 
Reviewed-by: Shubhrajyoti Datta <Shubhrajyoti.datta@xilinx.com>